Despite improvements in public health, advancements in vaccines, and the development of many classes of antibiotics, infectious disease is still responsible for over a quarter of all deaths worldwide. However, even for the most devastating of pandemics, history demonstrates a large variability in the severity and duration of infection. The long-term research goal of the candidate, Dr. Dennis Ko, is to determine the genetic basis for differences in susceptibility to bacterial infections. This knowledge is important for determining why some individuals are resistant to different infections and in developing therapies to decrease the mortality and morbidity of susceptible individuals. Experiments in this proposal will elucidate how human genetic differences contribute to variation in infection by the bacteria that causes typhoid fever, Salmonella typhi. The first aim is to identify human genetic differences that modify invasion of S. typhi into cultured cells. This is accomplished through a novel genetic association screen using cells obtained from hundreds of genotyped individuals. Genetic variants revealed by the screen will then be validated by measuring the effect of RNA interference on S. typhi invasion. The second aim is to determine how the identified genetic variants affect the cellular mechanism of S. typhi invasion. Fluorescence microscopy and flow cytometry will be used to determine if uptake of the bacteria or early establishment of an intracellular niche is being affected. Further experiments will include perturbation and assessment of molecules known to play roles in Salmonella invasion, including phosphoinositides, Rho-family GTPases, and secreted bacterial effectors. The third aim is to assess the relevance of identified differences on disease using mouse models and clinical association data. Mouse models will be used to determine if the identified genes affect establishment of infection and/or spread within the host. As a complement to these studies, the relevance of the genetic differences upon typhoid fever and other diseases will be addressed through case-control genetic association studies. The overall goal of these proposed experiments is discovery of both basic cell biological mechanism as well as human genetic variation important for health and disease. Dr. Ko, is a Life Sciences Research Foundation fellow in the laboratory of Dr. Samuel Miller. He received MD-PhD degrees from Stanford University. His thesis research on the lipid-storage disorder Niemann-Pick Type C provided a firm foundation in genetics and cell biology, and he has applied these disciplines to his current research on understanding human variation to bacterial infection. As a postdoctoral fellow, his efforts focused on developing a novel screening strategy for discovering genetic variants that modify Salmonella- induced cell death and determining how these variants affect human health. Dr. Ko plans to start an independent research lab studying susceptibility to S. typhi invasion, with the goal of expanding his focus to the importance of variation and adaptation on both sides of the host-pathogen relationship.